Echo suppressor



Sept. 17, 1929. D. MITCHELL ECHO SUPPRESSOR Filed March 13, 1928 1ATTORNEY Patented Sept. 17, 1929 UNITED STATES PATENT OFFICE DORENMITCHELL, OF NEW YORK, N. Y., ASSIGNOR TO AMERICAN TELEPHONE ANDTELEGRAPH COMPANY, CORPORATION OF NEW YORK I ECHO SUPPRESSOR Applicationfiled March 13, 1928. Serial No. 261,327.

This invention relates to echo suppressors, and, more particularly, toecho suppressors adapted for use in connection with two-wire telephonecircuits.

In two-wire telephone circuits employing repeaters of the so-called22-type,.it has been the practice to suppress the principal, echocurrents in the system by associating with each side of one of the22-type repeaters of 1n the circuit an echo suppressor unit to disablethe repeater path transmitting in the opposite direction. With such anarrangement, it has been found that false operation occurs due to echoeshaving little or no delay in transmission. For example, a currentpassing over one side of the QQ-repeater and open ating the suppressorunit associated with that side would be transmitted through the hybridcoil and back over the path in the opposite direction, therebyenergizing the suppressor unit associated withthe second path before thefirst echo suppressor unit would have time to disable the second path.

It has heretofore been proposed ,to over- 5 come this difficulty byinter osing a delay circuit between the input 0 a suppressor unitassociated with one side of the circuit and the input of the suppressorassociated with the other side of the circuit, thereby delaying thetransmission of the impulse around the circuit until the first echosuppressor unit has had time to operate. The use of such delay circuits,however, involves a considerable additional expense over and above theecho suppressor units proper.

Another scheme that has been proposed is to make the hang-over effect ofthe disabling relay-of each echo suppressor unit dependent, to someextent, upon the length and amplitude of theoperating impulse. With suchan arrangement, a voice wave passing over one side of the repeater andsetting the one suppressor unit into operationwould, after passing intothe return path to start the second suppressor unit into operation, beabruptly terminated by the operation of the first suppressor unit.Therefore, the resultant short impulse which might thus operate thesecond suppressor unit would not suflice to hold it operated for anylength of time, and

would not cause objectionable lockout of the speech. Experience hasshown, however, that such an arrangement, while effective to reducefalse operation to a considerable extent, accomplishes this result atthe expense of the quality of the speech transmitted, since somedistortion is introduced.

In accordance with the present invention, it is proposed to dispensewith the use of delay circuits and still retain the fundamental relayarrangement by arranging the echo suppressor units to exercise anelectrically interlocking (or opposing) control over the disablingequipment associated with the two sides of the circuit. In order toaccomplish this result, the operating winding of t e detector relay ofthe echo suppressor unit associated with each side of the circuit isconnected in series with a winding of the detector relay of the oppositesuppressor unit. The latter winding is connected in a reverse directionso that when either suppressor unit operates, it increases the biasingefl'ect on the relay of the other side by an amount depending upon themagnitude of the impulse and the number of turns in the reverse winding.Thus, if one echo suppressor unit is operated, it makes operation of thedetector relay of the other echo suppressor unit very difficult, due tothe increased bias.- Experience has shown that this interlcoking relayarrange- Inent gives substantially the same operation as would beobtained by the use of a delay circuit. Due to the fact that the initialvoice wave operates one relay and simultaneousl checks the operation ofthe other, very quic echoes are prevented from falsely operating theapparatus before the first relay can operate, and hence a delay circuitis not needed.

The invention will now be more fully understood from the followindescription when read in connection wit the accompanying drawing,Figures 1 and 2 of which show two different circuit arrangementsembodying the principles of the invention.

Referring to. Fig. 1, there'is shown a 22- repeater arrangementcomprising the path LE including an amplifier AE for transmitting in onedirection, and a path LW including an amplifier AW for transmitting inthe The echo suppressor equipment associated with the path LE comprisesan amplifier-detector arrangement DE of well-known type, the platecircuit of the detector of said arrangement being connected to thebattery B through the operating winding a of a detector relay RE. Thecircuit of the operating winding a also includes a winding 1; associatedwith the detector relay RW of the echo suppressor unit. associated withthe path LW.- Said last mentioned echo suppressor unit also includes anamplifier-detector arrangement DW, similar in all replate circuit of thedetector of the arrangement DW connected to the B battery through theoperating winding a of the detector relay RW. The winding 6 of thedetector relay RE is also included in series in the last describedcircuit. The Z) windings of-the two relays RE and RW are poledoppositely with respect to the a or operating "wii1dings associated witheach relay. Consequently, if

an o erating current flows from the amplifieretector through theoperating winding of the associated detector relay, a current flows atthe same time through the b winding ofthe opposite detector relay tobias the latter and prevent its response to impulses transmitted overone path of the repeater through'the hybrid coil to the other pathEachdetector relay is also biased by means of a 'windin c suppliedwith abiasing current from a attery B through suitable resistance. biasingwinding 0 is such as to normally oppose the action of current throughwinding 0.,

The detector relay RE controls a hang-over rela RE which performs thedual function of ort-circuiting (and thereby disabling) the path LW, andof introducing a hangover efl'ect by which the armature of the hang-overrelay maintains the short circuit for a predetermined hang-over periodafter the impulse which startedthe operation has ceased. The ban -overrelay RE comprises an operating win ing c, a hang-over winding a, andabias: ing winding b. The operating windin 0 is energized when thearmature of relay is removed from its back ,contact, thereby removing ashort circuit from the winding 0 so that current can flow from thebattery B". The biasing current for the winding b is sup- 66 plied bythe battery B and is regulated by the spects to the arrangement DE, withthe The normal current through theresistance 1', so that, under normalconditions, this biasing current is just suificient to hold the armatureof the hang-over relay RE firmly in its nonoperated condition. Thehang-over effect introduced by the winding a is controlled by the chargeand discharge of the condenser CE in a manner which will be described inmore detail later.

The detector relay RW associated with the amplifier-detector arrangementDW controls a hang-over relay RW', similar to RE. This hang-over relayalso includes an operating winding 0, a hang-over winding a and abiasing winding 6. The operation of the hangover winding 'a is alsocontrolled by the charge and discharge of the condenser CW. Likewise,the armature of the hang-over relay RW controls the application of ashort circuit to the path LE.

The operation is as follows: A voice wave incoming from the line Lpasses into the path LE and throughthe amplifier AE to the line L Partof the Voice energy passing over-the path LE enters theamplifier-detector arrangement DE, thereby producing a unidirectionalcurrent in the plate circuit of the latter. This current passes throughthe winding a of the detector relay RE and also through the opposingwinding 1) of the detector relay RlV. Both relays RE and RW are normallybiased by means of the current through their windings a so as to holdtheir armatures in the non-operated condition, as shown in the diagram.The current through the winding a of the relay RE produces an effect ofsuch magnitude as to overcome the normal bias due 'to the winding 0 andtoshift the armature of the relay from its nonoperated to its operatedcontact. The current through the winding 1) of the relay RW produces apull in the same direction as the normal bias due to the current in thewinding 0, and, consequently, the armature of the relay RW will be heldin'its non-operated posi-- tion notwithstanding the fact that some ofthe voice energy flowing over the path LE may pass from the hybrid coilinto the path LWV to operate the amplifier-detector arrangement DWbefore relay RE- is operated, and thereby energize the winding 0, of thedetector relay RW. Obviously, this prevents any false operation ofthe'echo suppressor unit associated with the path LVV.

Returning to the detector relay RE, as soon as its armature is shiftedfrom its non-operated contact, it opens the short circuit to ground,which normally prevents the energization of the winding dot thehang-over relay RE, and said winding is at once energized to shift thearmature of relay RE to its operated contact and thereby apply a shortcircult to the. path LW and to the input of the amplifier-detector DW.iransniisison from the line L to the line L is thereby'prevented', sothat echoes passing over the circuit in the direction from the listeningsubscriber tothe talking subscriber are blocked.

The armature of the detector relay RE, upon reaching its operatedposition, provides a discharge path through the resistance r, to groundfor the condenser CE, said condenser being previously charged by thebattery B through the hang-over windlng a and the resistance 1-,.

When the voice wave passing over the path LE ceases, winding a of relayRE is .no longer energized, and the'pull due to the biasing winding 0 ofsaid relay restores the armature to its non-operated position. Theadditional bias introduced into the relay RW due to the current flowthrough the winding b of the relay RW also ceases, so that the relay RWis in a condition to res 0nd to voice waves passing over the circuit W.

As soon as the armature of the relay RE leaves its operated positionupon being restored to normal, the discharge path to ground'for thecondenser CE is opened, and current flows from the battery B through thewinding a and resistance r, to charge the condenser vCE. .This chargingcurrent passes through the .Winding a in such a direction as to tend tohold the armature of the hang-over relay'RE in-its operated position andthereby maintain the-shortcircuit closed. As the con- 7 denser OEcharges up, the current through the winding a decreases in accordancewith the exponential law until the pull due to the biasing winding 1) issufficient to overcome the pull due to the winding a, whereupon the ar-"mature of the hang-over relay RE is restored to its non-operatedposition and the short circuit is removed from the line LW. Thishang-over period can be made sufliciently long to insure that any echocurrents returning from east to west will have arrived at the foregoingpoint before the short circuit is removed.

It will, of course, be obvious that for voice currents transmitted fromthe line L over the path LW to the line L,, the operation of the otherhalf ofv the a" paratus including the amplifier-detector D the detectorrelay RW and the hang-over relay RW, will be similar to that alreadydescribed. In this case, of course, the unidirectional current from DW'energizes the winding (1 of the relay RW to shift its contact. At thesame,

time, the current passes throughthe op osing winding 6 of the relay REto increase t e has of the latter, tending to hold thearmature againstits back contact.

In the arr ement above described," relaysRE, RE, W and RW' may all be exactly alike, the differences in operation being due to the differentcircuit arrangements by which they are connected. A modifiedarrangemen't is ossible, however, as shown in Fig. 2, in whic thefunction of the opposing winding b and the biasing winding, 0 may bethat plate current from a given amplifier-detector flows through theoperatin Winding of its associated relay and the win ing be of theopposite relay. Consequently, when one relay, as, for example, RE, isoperated by its winding a in response to voice currents, the biasingcurrent throu h the winding be of the opposite relay R\ is increased,thereby preventing operation of the latter. The circuits of thehang-over relays RE and RW' are identical with and are operated in thesame manner as those of Fig. 1.

It will be obvious that the general principles herein disclosed may beembodied in many other organizations widely difierent from thoseillustrated without de arting from the spirit of the invention as de nedin the following claims.

What is claimed is:

1. In a two-way communication system, a

directions, an echo suppressor unit associated with each path comprisinga detector anda relay operated therehy,.each relay controlling means fordisahling the opposite path, and an opposing winding for each relayenergized under the control of the detector which operates the oppositerelay, whereby when one relay is energized for operation the ther relayis prevented from operating.

2. In a two-way communication system, a pair of paths adapted totransmit in opposite directions, an echo suppressor unit associated witheach path comprisin a detector and a relay operated thereby, eac relaycontrolling means .for disabling the opposite path, and a winding foreach relay connected in opposition to its normal windinggand energizedby current supplied by the detector which operates the opposite relay,whereby when one relay is energized for operation the other relay isprevented from operation.

3. In a two-way communication system, a pair of paths adapted totransmit in opposite directions, an echo suppressor unit associated witheach path comprising a detector and a relay operated thereb ,each relaycontrolling means for disabling t e opposite path, an op eratingwinding-and an opposing winding for relay being in circuit with lay isprevented from operation.

4. In a-two-way communication system, a

pair of paths adapted to transmit in opposite pair of paths adaptedtotransmit in opposite directions, an echo suppressor unit associatedwith each path comprising a detector and a relay operated thereby, eachrelay controlling means for disabling the opposite path, an operatingwinding and an opposing winding for each relay, the operating winding ofone relay and the opposing winding of the other relay being connected tobe supplied with current from one detector, and the operating winding ofthe latter relay and opposing winding of the first relay being suppliedwith current from the other detector, whereby when one relay isenergized for operation, the

. other relay is prevented from operation.

5. In a two-way communication system, a pair of paths adapted totransmit in opposite directions, an echo suppressor unit associated witheach path comprising a detector and a relay operated thereby, anopposing winding for each relay energized under the control of thedetector which operates the opposite relay, whereby when one relay isenergized for operation, the other relay is prevented from operation,means controlled. by each relay for disabling the opposite path, andmeans for maintaining the path disabled a predetermined time after thetransmission wave passing over the other path has ceased.

- 6. In a two-way communication system, a pair of paths adapted totransmit in opposite directions, an echo suppressor unit associated witheach path comprising a detector and a relay operated thereby, anopposing winding for each relay energized by current supplied by thedetector which operates the opposite relay, whereby when one relay isenergized for operation, the other relay is prevented from operation,means controlled by each relay for disabling the opposite path, andmeans for maintaining the path disabled'a predetermined time after thetransmission wave passing over the other path has ceased.

posing winding of the other relay being connected to be supplied withcurrent from one detector, and the operating winding of the latter relayand opposing winding of the first relay being supplied with current fromthe other detector, whereby when one relay is energized for operation,the other relay is prevented from operation, means controlled by eachrelay for disabling the opposite path, and means for maintaining thepath disabled a predetermined time after the transmission wave passingover the other path has ceased.

9. In a two-way communication system, a pair of paths adapted totransmit in opposite directions, an echo suppressor unit associated witheach path comprising a detector and a relay operated thereby, each relaycontrolling means for disabling tne opposite pat-h, an opposing windingfor each relay, and means whereby when one relay is operated theopposing winding of the other relay is simultaneously energized toprevent the operation of said other relay and thereby suppress quickechoes.

In testimony whereof, I have signed my name to this specification this12th day of March, 1928.

DOREN MITCHELL.

7. In a two-Way communication system, a

- pair of paths adapted to transmit in opposite directions, an echosuppressor unit associated with each path comprising a detector and arelay operated thereby, an operating winding and an opposing winding foreach relay, the said opposing winding of each relay being in circuitwith the operating winding of the other relay, whereby when one relay isenergized for operation, the other relay is prevented from operation,means controlled by each relayfor disablin the opposite path, and meansfor maintaining the path disabled a predetermined time after thetransmission wave passing over the other path has ceased.

8. Ina two-way communication system, a pair of paths adapted to transmitin opposite directions, an echo suppressor unit associated with eachpathcomprising a detector and a relay operated thereby, an operatingwinding and an opposing winding for each relay, the operating windingfor one relay and the op-

